What is a blue moon exactly? The moon?s phases don?t coincide perfectly with the Julian calendar we use, with 30 or 31 days to each month, which is why every so often there are actually thirteen full moons in a year rather than the expected twelve. When there are two full moons in a month the second one is always referred to as a ?blue moon?, hence the expression ?once in a blue moon?. For those who believe such things, the energy is considered to be especially strong during a blue moon. When there are two dark moons in a month, the second one is called the ?black moon?, and again magic is thought to be very powerful at this time. The dark moon is the time for banishing spells and bringing things to a close. The full moon is a time of creativity and fulfilment, and also healing.